fix wrong heading level (rust-lang#1573)

Author: tshepang

src/doc/rustc-dev-guide/src/building/bootstrapping.md

@@ -407,7 +407,7 @@ usually means something is quite wrong -- or you're trying to compile e.g.
 the unlikely case that you actually need to invoke rustc in such a situation,
 you can tell the bootstrap shim to print all env variables by adding `-vvv` to your `x.py` command.
 
-### Directories and artifacts generated by `bootstrap`
+## Directories and artifacts generated by `bootstrap`
 
 This is an incomplete reference for the outputs generated by bootstrap: